Carbon fibers are generally defined as a fiber containing at least about 92 wt-% of carbon. Carbon fibers containing 99 wt-% or more of carbon are often referred to as graphite fibers. Carbon fibers (CFs) are used in various applications owing to their excellent tensile properties, thermal and chemical stabilities (in absence of oxidizing agents) and thermal and electrical conductivities. The conventional applications of CFs include aircraft frames, turbine blades, automobile panels, sporting goods and industrial components.
Currently, the carbon fiber market is dominated by carbon fiber derived from polyacrylonitrile (PAN), with the balance being made up of fibers from pitch and rayon. CFs with distinct properties result from the processing of different precursor fibers. In a typical process in the art for converting organic polymer fibers into carbon fibers, the organic polymer fiber is first heat-stabilized in air in an oxidation process conducted at a temperature of 200 to 400° C. The thus stabilized precursor fibers then undergo controlled pyrolysis, i.e., a carbonization step, comprising heat-treating in an inert atmosphere such as nitrogen to a temperature of from about 300° C. to about 3000° C., which removes non-carbon elements such as hydrogen, oxygen and nitrogen from the oxidized fiber. It is known in the art that heating at the higher end of the temperature spectrum, e.g., between about 1000° C. and about 3000° C. may achieve higher carbon content, thereby producing CFs with higher Young's modulus values.
For automotive applications, desired mechanical properties for carbon fibers include tensile strength of >1.72 GPa, tensile modulus of >172 GPa and elongation at break of about 1%.
In addition to the limited mechanical properties of conventional CFs, the currently used methods of preparing CFs can be costly. For example, the cost of the precursor fiber amounts to approximately 40% to 50% of the total cost of preparing the carbon fiber. Therefore, there is a need in the art for lower cost precursor fibers that yield carbon fibers of excellent quality would significantly reduce the cost of CFs. An additional benefit would be to enable the expansion of CF applications to industries and markets such as those related to the automotive industry.
Furthermore, it is desirable to provide a source of carbon fibers that derives from a renewable source that does not contribute to global warming.
Polysaccharides have been known since the dawn of civilization, primarily in the form of cellulose, a polymer formed from glucose by natural processes via β(1→4) glycoside linkages; see, for example, Applied Fibre Science, F. Happey, Ed., Chapter 8, E. Atkins, Academic Press, New York, 1979. Numerous other polysaccharide polymers are also disclosed therein.
Only cellulose among the many known polysaccharides has achieved commercial prominence as a fiber. In particular, cotton, a highly pure form of naturally occurring cellulose, is well-known for its beneficial attributes in textile applications.
It is further known that cellulose exhibits sufficient chain extension and backbone rigidity in solution to form liquid crystalline solutions; see, for example O'Brien, U.S. Pat. No. 4,501,886. The teachings of the art suggest that sufficient polysaccharide chain extension could be achieved only in β(1→4) linked polysaccharides and that any significant deviation from that backbone geometry would lower the molecular aspect ratio below that required for the formation of an ordered phase.
More recently, glucan polymer, characterized by α(1→3) glycoside linkages, has been isolated by contacting an aqueous solution of sucrose with GtfJ glucosyltransferase isolated from Streptococcus salivarius, Simpson et al., Microbiology, vol 141, pp. 1451-1460 (1995). Highly crystalline, highly oriented, low molecular weight films of α(1→3)-D-glucan have been fabricated for the purposes of x-ray diffraction analysis, Ogawa et al., Fiber Diffraction Methods, 47, pp. 353-362 (1980). In Ogawa, the insoluble glucan polymer is acetylated, the acetylated glucan dissolved to form a 5% solution in chloroform and the solution cast into a film. The film is then subjected to stretching in glycerine at 150° C. which orients the film and stretches it to a length 6.5 times the original length of the solution cast film. After stretching, the film is deacetylated and crystallized by annealing in superheated water at 140° C. in a pressure vessel. It is well-known in the art that exposure of polysaccharides to such a hot aqueous environment results in chain cleavage and loss of molecular weight, with concomitant degradation of mechanical properties.
Polysaccharides based on glucose and glucose itself are particularly important because of their prominent role in photosynthesis and metabolic processes. Cellulose and starch, both based on molecular chains of polyanhydroglucose are the most abundant polymers on earth and are of great commercial importance. Such polymers offer materials that are environmentally benign throughout their entire life cycle and are constructed from renewable energy and raw materials sources.
The term “glucan” is a term of art that refers to a polysaccharide comprising beta-D-glucose monomer units that are linked in eight possible ways, Cellulose is a glucan.
Within a glucan polymer, the repeating monomeric units can be linked in a variety of configurations following an enchainment pattern. The nature of the enchainment pattern depends, in part, on how the ring closes when an aldohexose ring closes to form a hemiacetal. The open chain form of glucose (an aldohexose) has four asymmetric centers (see below). Hence there are 24 or 16 possible open chain forms of which D and L glucose are two. When the ring is closed, a new asymmetric center is created at C1 thus making 5 asymmetric carbons. Depending on how the ring closes, for glucose, α(1→4)-linked polymer, e.g. starch, or β(1→4)-linked polymer, e.g. cellulose, can be formed upon further condensation to polymer. The configuration at C1 in the polymer determines whether it is an alpha or beta linked polymer, and the numbers in parenthesis following alpha or beta refer to the carbon atoms through which enchainment takes place.

The properties exhibited by a glucan polymer are determined by the enchainment pattern. For example, the very different properties of cellulose and starch are determined by the respective nature of their enchainment patterns. Starch or amylose consists of α(1→4) linked glucose and does not form fibers among other things because it is swollen or dissolved by water. On the other hand, cellulose consists of β(1→4) linked glucose, and makes an excellent structural material being both crystalline and hydrophobic, and is commonly used for textile applications as cotton fiber, as well as for structures in the form of wood.
Like other natural fibers, cotton has evolved under constraints wherein the polysaccharide structure and physical properties have not been optimized for textile uses. In particular, cotton fiber is of short fiber length, limited variation in cross section and fiber fineness and is produced in a highly labor and land intensive process.
O'Brien, U.S. Pat. No. 7,000,000 discloses a process for preparing fiber from liquid crystalline solutions of acetylated poly(α(1→3) glucan). The thus prepared fiber was then de-acetylated resulting in a fiber of poly(α(1→3) glucan).
The inventive method described herein, results in carbon fibers meeting these desired mechanical benchmarks and would further reduce the costs making CFs available to additional industrial sectors.